Building an SZBK07-based thermostat for Peltier cooling

In this video I show you my DIY thermostat. The thermostat is based on the popular SZBK07 DC-DC converter which provides power for a Peltier cooler. The SZBK07 DC-DC converter is controlled by a MCP41100 8-bit digital potentiometer. The whole thermostat is based on a self-regulating feedback logic. The cold side temperature of the Peltier cooler is measured with an NTC thermistor and based on the relationship between the measured temperature and the goal temperature the power for the Peltier cooler is either increased (T(goal) is smaller than T(measured)) or decreased (T(goal) is higher than T(measured)). The power remains unchanged if the temperature stays within a certain tolerance. The theoretical step size for the temperature ("resolution") is around 0.21°C.
For a more accurate control, check this video of mine: • MCP4725 12 bit DAC and...
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[@MethiasZa]

Thanks, fantastic video. Exactly along the lines of what I was thinking (I'll replace the input sections to be written from a PC so I can remotely control it, but in theory its the same logic). Juts need to finish watching your videos on the Peltiers so I know what I'm doing there and can start ordering parts.

[@CuriousScientist]

Thank you very much! I remember you mentioned that coding is not an issue for you, however if I can recommend, look at my most recent ADS1256 video. Just for some ideas. I show there a quite simple way of controlling the STM32/Arduino via the serial port. It is straightforward and simple. If you need some extra info for the parts before you order them, let me know, maybe I can help you. Cheers!

[@MethiasZa]

@@CuriousScientist Thanks, I've started watching it and then work happened which gets in the way of planning projects. Biggest unknown is which peltiers to get. I can get the 12706's reasonably easily locally, the banggood will deliver here but our postal service is so shocking they may never arrive. I'm trying to see if I can get anything else, just a concern its not enough power. I've found so far the DC-DC buck converter, STM32 blue pill (its nice and small, just need to see if I can find the ST Link to put the arduino boot loader on it).

[@CuriousScientist]

If you would use the exact or similar parts, I recommend you to check my website and check the page called "Tools". I took good quality pictures of each products and provided links for them (mostly Banggood or Aliexpress). The links are affiliate links, but of course you are not forced to use them. But it might be useful to at least look up the product. For this camera project, I would think about choosing either the TEC12708 or the TEC12710, and more than one piece. Probably I would go for 2-4 pieces, so they could run at lower power but their overall cooling power would be high enough, and the contribution of the Joule heating would be lower (easier to cool). They provide higher cooling power than the TEC12706 and it is still reasonable to keep their hot side cold. Obviously, cooling power-wise, the TEC12715 would be the best, but its heat dissipation alone can be higher than 200 W, which is quite a challenge to dissipate.

[@MethiasZa]

@@CuriousScientist I'm sure I saw a 12709 whilst looking for parts. Will see if I can get something around there. I'm trying to avoid too much hot side power to dissipate as the heatsink will need to be supported on the focuser of the telescope, the less weight there is there the better to avoid problems of tilt. Ye, I went to your website and I'm using that to find similar parts here.

[@MethiasZa]

@@CuriousScientist Me again :). One thing im still unsure on is, which is better, stacked peltiers (2x12709's) or 2 peltiers (12709's) in parallel. Which configuration should be able better transfer heat and obtain a lower Dt?

[@iridescentstarling]

I have built this setup but I'm seeing strange behavior: when I read the output voltage of the MCP41100 without the SZBK07 connected, it behaves as expected: the output voltage changes linearly (0-5V) with the wiper position value (0-255). However, as soon as I connect the SZBK07, the middle part of the wiper positions have a very small effect on the output voltage, so for example 69 corresponds with 2.0V and 216 with 1.0V. First and last positions change the voltage vary rapidly. I did leave the original trimmer in place, not sure if that's relevant? Have you observed this behavior too? I see you have another setup using the MCP4725, have you measured the linearity of the injection voltage of that circuit?

[@CuriousScientist]

I haven't checked the linearity on purpose, but as far as I remember from the tests, it behaved fairly linear. I cannot promise anything, but I might test it in the future. Did you make sure that all the grounds are connected? If there's a missing link between the grounds, the circuit can behave erroneously.

[@iridescentstarling]

@@CuriousScientist yes, all grounds are connected. I'll give the MCP4725 a go and will report back here!

[@iridescentstarling]

Doing some further tests, it seems that the non-linearity of the MCP41100 is resulting from the 10k resistor used (perhaps the current is too close to the max?). I tested with 67,7k and 100k and it was increasingly linear with a higher resistor value. However, with these higher resistors, the SZBK07 also responds in much smaller steps. With 10k I have a very good range (3.7V to 24V), but with 100k the range is decreased to 21V to 24V, even though the injected voltage range is the same for both setups (so it seems to be more sensitive to current than voltage?)

[@CuriousScientist]

Hmm, interesting. The non-linearity could be related to the current perhaps, yes. Please note that the potentiometer in the circuit is a rheostat! It is not controlling voltage, but current! So when we inject voltage, we inject current actually. So if you increase the resistor's value, you limit the injected current, therefore you won't have the same range. Try it with the 12-bit ADC like I did, it is a much better approach.

[@iridescentstarling]

I did some further tests, I wanted to rule out issues with the chip itself (which I had been using for some time now), and I used a fresh one (both were purchased from Mouser, so not a fake). I had the same results on both chips. They seemed linear at first, but after ~10V (on a 0-25.5V scale) it leveled off and hardly increased. All grounds were connected. I also plotted the values from your video and it seems linear but it's hard to tell because you only go up to 5V here and in my test that's not at all an issue either. I don't know your input voltage or the resistor value used (or whether the caps make any difference), so I can't replicate it here. With the same controller and power supplies and the MCP4725 it was perfectly linear. Maybe it's a good idea to link to the the MCP4725 at the beginning of this video and the other one about controlling the SZBK07, mentioning this new & improved method? In any case, thank you so much for sharing all this valuable information with us!

[@gregmarkowski2181]

im a bit confused by the linear potentiometer. What is its purpose? the schematic on your website does not include one.

[@gregmarkowski2181]

dumb question just saw your other video

[@CuriousScientist]

:) No worries! At least you took the time and looked around on my channel! I hope you found what you were looking for. Have a great day!

[@oneplus2896]

Is it hot or does it last 24 hours?

[@CuriousScientist]

Depends on the power you use. You can use it all day long, but if you want to run it at high current, it is recommended to add a fan and actively cool the electronics.

[@MeisterQ]

Why dont you use a PWM controller?

[@CuriousScientist]

Because Peltier coolers are not really compatible with PWM.

[@MeisterQ]

@@CuriousScientist could you make a video about how peltiers act when you try to control them with pwm?

[@CuriousScientist]

Sorry, but I don't make videos on request, unless you want to sponsor me. I have a huge backlog of projects and I cannot afford to work on requests for free. I hope you understand. Just read the conclusion of the pros if you don't believe me: www.ti.com/lit/an/slua979a/slua979a.pdf

[@MeisterQ]

@@CuriousScientist im believing you and I understand. Relax :) thanks for your work. I like your stuff about the peltiers

[@CuriousScientist]

No worries! ;) More Peltier-related videos will come very soon. Stay tuned!